1. Field of the Invention
The present invention relates to a two-terminal nonlinear element, a method for fabricating the same, and a liquid crystal display device using the two-terminal nonlinear element as a driving element.
2. Description of the Related Art
In recent years, liquid crystal display devices have been used as display devices for personal computers, wordprocessors, terminals for office automation, TV sets, and the like since they consume less power and are thin and light in weight. When used for these apparatuses, the liquid crystal display devices must provide a larger capacity display and a higher quality. The liquid crystal display devices are driven by a simple matrix driving method by voltage averaging using TN (twisted nematic) or STN (super twisted nematic) type liquid crystal. This method and type, however, fail to provide a sufficient contrast when the number of scanning lines increases and thus are not suitable for a large capacity display. To overcome this problem, an active matrix driving method has been realized where a display screen is divided into pixels in a matrix and pixel electrodes in respective pixels are connected with individual switching elements. As such switching elements, thin film transistors (TFTs) using amorphous silicon semiconductors and two-terminal nonlinear elements utilizing nonlinear resistance characteristics are used. The two-terminal nonlinear elements are especially promising since liquid crystal display devices using the two-terminal nonlinear elements are simple in structure and thus advantageous in production cost. In particular, a two-terminal nonlinear element having a metal-insulator-metal (MIM) structure (hereinafter, such a two-terminal nonlinear element is referred to as an "MIM element") has already been put into practical use.
FIG. 6 is a structural view of a liquid crystal display device using MIM elements, illustrating the inside of a portion thereof roughly corresponding to one pixel by removing part of a counter substrate. FIG. 7 is a sectional view of a substrate where MIM elements are formed (hereinbelow, such a substrate is referred to as an "MIM-element substrate") taken along line A-A' of FIG. 6.
The MIM-element substrate includes signal lines 3, MIM elements 4, pixel electrodes 5, and the like formed on a glass substrate 1. The counter substrate includes strip-shaped counter electrodes 6 formed on a glass substrate 2 in a direction perpendicular to the signal lines 3. These substrates are laminated together with spacers (not shown) having a predetermined size interposed therebetween, so that they are accurately positioned with respect to each other while securing a space of several micrometers to several tens of micrometers therebetween. A liquid crystal material is then injected in the space between the substrates, thereby to complete the liquid crystal display device. Each of regions where the pixel electrodes 5 and the counter electrodes 6 face each other constitutes a pixel.
Each MIM element 4 is a two-terminal nonlinear element composed of a lower electrode 3a as an extension of the signal line 3, an insulating film 7 formed on the lower electrode 3a, and an upper electrode 8 formed on the insulating film 7. The lower electrode 3a is made of tantalum (Ta), the insulating film 7 is made of tantalum oxide (TaO.sub.x ), and the upper electrode 8 is made of chromium (Cr), titanium (Ti), or aluminum (Al). The pixel electrode 5 and the counter electrode 6 are made of a transparent conductive material such as ITO (indium tin oxide). Anodic oxidation is often used to form the insulating film 7 made of tantalum oxide. This is preferable because the thickness of the insulating film 7 which may greatly affect the device characteristics can be controlled by varying the formation voltage.
An alignment film 10 is formed over the MIM elements 4 and the pixel electrodes 5 and rubbed in a predetermined direction. An alignment film is also formed over the counter electrodes 6 and rubbed in a predetermined direction. The orientation state of liquid crystal display molecules is determined by the two alignment films. Optical films are attached to the glass substrates 1 and 2 so that a predetermined optical mode can be obtained. When a TN type liquid crystal material is used in a normally white mode, polarizing plates 11 and 12 are attached to the outer surfaces of the glass substrates 1 and 2, respectively.
The MIM element 4 exhibits nonlinear resistance characteristics where the resistance becomes high when a voltage applied between the lower electrode 3a and the upper electrode 8 is low and becomes low when the voltage is high. The liquid crystal display device utilizes the nonlinear resistance characteristics of the MIM elements as the switching characteristics at the application of voltages to the pixel electrodes 5 and the counter electrodes 6, so as to change the orientation state of the liquid crystal molecules. A liquid crystal display device with a higher display quality is obtained as the ratio of the capacitance of the liquid crystal layer (C.sub.LC) to the capacitance of the MIM element (CM.sub.MIM) is larger when the MIM element is driven. Therefore, the ratio is normally designed to be about 10 or more.
Such a liquid crystal display device using the MIM elements with the above configuration is advantageous in that the number of production steps is small compared with the case of using TFTs. It has the problems however that size reduction of the MIM elements is difficult and that the insulation of the MIM elements tends to be easily broken. The size reduction of the MIM elements is required as the pitch of the pixels is reduced to effect a large capacity display. That is, in order to secure the ratio of the capacitance of the liquid crystal layer to the capacitance of the MIM element of about 10 or more, the area of the MIM element must be reduced. The capacitance of the MIM element (C.sub.MIM) may also be reduced by increasing the thickness of the insulating film 7. However, this degrades the sharpness in the voltage-current characteristics of the MIM element and thus is not recommendable. It is therefore required to reduce the area of the MIM element.
The insulating film 7 is made as thin as about 400 to 700 .ANG. to obtain the required device characteristics. This reduces the electrical resistance and thus tends to easily permit the insulation to break. A pixel having an insulation-broken MIM element does not operate normally, generating a point defect.
In order to solve the above problems, Japanese Laid-Open Publication No. 3-26367, for example, proposes a so-called lateral structure where a side wall of the lower electrode is used for the MIM element to reduce the size of the MIM element. A top contact structure utilizing a through hole for preventing the insulation break is also proposed.
The lateral structure will be described with reference to FIGS. 8A and 8B. FIG. 8A is a plan view of one pixel of an MIM-element substrate of a liquid crystal display device using the lateral structure. FIG. 8B is a sectional view taken along line B-B' of FIG. 8A.
The liquid crystal display device shown in FIGS. 8A and 8B includes signal lines 3 and pixel electrodes 5 formed on a glass substrate 1 as in the liquid crystal displayed device shown in FIG. 6. In this conventional liquid crystal display device, however, each MIM element 4 is formed at a side wall of the signal line 3. More specifically, a thinner insulating film 7a having nonlinearity is formed on a side wall of the signal line 3, while a thicker insulating film 7b which does not constitute the MIM element is formed on the top surface of the signal line 3. An upper electrode 8 is formed on the thinner insulating film 7a. Thus, the side wall of the signal line 3, the thinner insulating film 7a, and the upper electrode 8 constitute the MIM element. Since the thinner insulating film 7a having nonlinearity is formed on a side wall of the signal line 3 of which thickness is about 1000 .ANG., the area of the MIM element is determined by the product of the thickness of the signal line 3 and the width of the upper electrode 8. This makes it possible to fabricate an MIM element much smaller than the limit of exposure precision in photolithography (about several micrometers), reducing the substantial capacitance of the MIM element (C.sub.MIM). As a result, the ratio of the capacitance of the liquid crystal layer (C.sub.LC) to the capacitance of the MIM element (C.sub.MIM) increases even if the patterning precision of the upper electrode is the same.
The top contact structure will be described with reference to FIGS. 9A and 9B. FIG. 9A is a plan view of one pixel of an MIM-element substrate of a liquid crystal display device using the top contact structure. FIG. 9B is a sectional view taken along line C-C' of FIG. 9A.
The insulation break of an MIM element is considered to occur because an electric field is concentrated on an etching end of a lower electrode which serves as part of the nonlinear element and the quality of an insulating film formed over the etching end of the lower electrode is poor. Moreover, since such a poor insulating film over the etching end constitutes the MIM element, the resultant device characteristics tend to be varied. In the top contact structure, the etching end of the lower electrode is not used as part of the MIM element.
One pixel of the liquid crystal display device shown in FIGS. 9A and 9B includes a lower electrode 3a as an extension of a signal line 3, a thinner insulating film 7a having nonlinearity, an upper electrode 8, and a pixel electrode 5 as in the liquid crystal display device shown in FIG. 6. In this conventional liquid crystal display device, however, the thinner insulating film 7a is formed in the bottom of a through hole of a thicker insulating film 13, i.e., surrounded by the thicker insulating film 13, and connected with the upper electrode 8 in the through hole. According to this structure, the etching ends of the lower electrode 3a are not used as part of the MIM element, but are protected with the thicker insulating film 13. This prevents an electric field from concentrating on the etching ends of the lower electrode, reducing point defects and thus improving the production yield.
As described above, the lateral structure realizes the size reduction of MIM elements, and the top contact structure prevents an occurrence of insulation break and reduces the variation in device characteristics. However, there has not been realized a structure where the above two structures are combined to satisfy the size reduction of MIM elements, the prevention of an occurrence of insulation break, and the unification of device characteristics simultaneously. This is because a side wall of the lower electrode is used as a component of the MIM element in the lateral structure, while a portion of the top surface of the lower electrode is used as a component of the MIM element in the top contact structure.